1. Field of the Invention
The present invention relates to a light-transmissive resin sealed semiconductor and a production process thereof, and specifically relates to a solar cell module and a production process thereof. The solar cell nodule comprises a photoelectric transducer in which a semiconductor photoactive layer as a light converting element and a conductive layer are formed on a substrate having a conductive surface. The solar cell module is particularly excellent in heat resistance and fire retardancy.
2. Related Background Art
Nowadays, environmental problems have been increasingly noted worldwide. In particular, global warming caused by CO.sub.2 emission has been a serious concern and the need for clean energy that does not exhaust CO.sub.2 increases. Solar cells are expected to be useful as a clean energy source because of their safety and ease in handling.
There are various types of solar cells; typical examples include crystalline silicon solar cells, polycrystalline silicon solar cells, amorphous silicon type solar cells, copper-indium-selenide solar cells, and compound semiconductor solar cells. Among them, thin film crystalline silicon solar cells, compound semiconductor solar cells, and amorphous silicon type solar cells have been active subjects of R&D since large area cells may be realized at a relatively low cost.
In particular, a thin film solar cell, typically represented by an amorphous silicon type solar cell in which silicon is deposited on a metal substrate having a conductive surface and a transparent conductive layer is formed thereon, is light in weight and excellent in impact resistance and flexibility, and offers hope to be useful in module form from among the above mentioned solar cells. A solar cell module that is flexible requires protection of the internal solar cells by covering the surface of the incident light side thereof with a transparent covering material, unlike the case of silicon deposition on a glass substrate.
As for such surface covering material, a construction may be considered wherein a transparent thin film of a fluoride polymer such as a fluororesin film or fluororesin coating is provided on the topmost surface and various transparent organic thermoplastics are provided inwardly thereof. This construction is based on the facts that a fluoride polymer is excellent in weatherability and water repellency, thereby making smaller the reduction of conversion efficiency of the solar cell module caused by decrease of the light transmittance due to yellowing, cloudiness, or fouling of the surface and that a transparent organic thermoplastic is inexpensive and a large amount may be used as a filler protecting the photovoltaic element which is made of a semiconductor.
FIG. 6 shows an example of a conventional solar cell module. This solar cell module comprises a thin film layer 601 of a fluoride polymer, a thermoplastic transparent organic resin 602, a photovoltaic element 603, and an insulating layer 604. In the construction of this solar cell module, an organic resin such as is used for the light receiving surface is also used at the back surface.
More specifically, fluoride polymer thin film layer 601 is composed of a fluororesin film such as an ethylene-tetrafluoroethylene copolymer (ETFE) film or polyvinyl fluoride (PVP) film; thermoplastic transparent organic resin 602 is produced from ethylene-vinyl acetate copolymer (EVA), butyral resin or the like; and insulating layer 604 is selected from various organic resin films including nylon film and aluminum laminated Tedlar film. In this solar cell module, thermoplastic transparent organic resin 602 functions as the adhesive that idheres photovoltaic element 603 to fluororesin film 601 and to insulating layer 604 and as the filler that protects the solar cells against scratches and impacts.
However, in a solar cell module with such structure as described above having a surface covering material, the thermoplastic transparent organic resin used as the filler becomes cloudy due to partial gelation of the resin during outdoor exposure for as long as 20 years or yellowing occurs in the resin due to an increase in the number of conjugated double bonds; these inevitably cause a decrease of light transmittance of the resin and conversion efficiency of the solar cell module. This problem is more serious in applications where the module is incorporated with roofing or other construction material and the module temperature is more elevated.
Furthermore, acceleration of the yellowing is known when the module is subjected to temperature conditions exceeding 80 degrees or higher in the case of EVA filler, for example. In the case of butyral resin filler, hygroscopicity is relatively high and moisture easily attacks defective parts of the photovoltaic elements. The moisture and electric field of photovoltaic element might cause a metal compound of the collecting electrode or the like to be grown by repetition of ionization and precipitation; when such reactions proceed further, short circuits may be formed among the photovoltaic elements and the generated electric current becomes unable to be led outward, thereby lowering the conversion efficiency. Butyral resin has an additional problem that the transparency is lost under high temperature and high humidity conditions.
For overcoming these problems, Japanese Patent Publication No. 4-76229 discloses a protective film including a resin derivative which comprises a perfluoroalkylene group and active hydrogen for a CdS/CdTe type solar cell formed on a substrate. As for the resin comprising a perfluoroalkylene group and active hydrogen, a product known by the trade name of Lumiflon (Asahi Glass Co., Ltd.) is mentioned. According to Japanese Patent Publication No. 4-76229, Lumiflon is a fluorine-containing polymer having a number average molecular weight from 20,000 to 80,000 and contains perfluoroalkylene groups and pendant active hydrogens (more specifically OH group), which produce a cross-linked polymer (derivative) by reaction with melamine or a compound having an isocyanate group.
In addition, Japanese Patent Publication No. 4-76229 also discloses, in the description of the example, a protective film which is excellent in moisture resistance and is obtained by cross-linking Lumiflon with an isocyanate or a resol type phenolic resin. However, the coating process disclosed therein requires placement of the coating on the topmost surface of a solar cell; however, the pot life of the resin that has been mixed with the mentioned cross-linking agent is generally short and no one is known having a long after the cross-linking agent is mixed in. In practice, the pot life is extended by protecting the isocyanate with a blocking agent. However, adoption of the coating structure, as mentioned above, where the surface film is laminated to the resin causes a problem that the cross-linking reaction does not proceed because the blocking agent is not dissociated and, thus, not volatilized during resin cross-linking.
On the other hand, lamination of the surface film after the cross-linking of resin is difficult since the cross-linked product lacks tackiness and adhesiveness. Furthermore, when the cross-linking agent is melamine, no effective blocking agents are known. Now, the resin mentioned above should be used on the topmost surface of the solar cell module. However, the surface hardness of the resin is low and it is easily damaged by outdoor sand and dust, which will accumulate on the damaged part; thereby the sunlight might be shielded. Depending on the manner of the resin lamination, simple application of the coating might create pin holes and inclusion of dust; moisture and oxygen thus may penetrate into the photovoltaic element. It furthermore is known that thick coating with a material having rubber elasticity to protect a solar cell element is effective for preventing damage of the solar cell element caused by bending and difference of thermal expansion due to change of temperature; however, making the coating resin thick is substantially difficult, may damage the element, and does net giver sufficient protection for concave and convex parts in the solar cell surface brought by electrical wiring, etc. In addition, Lumiflon mentioned above does not have rubber elasticity.
Thus, organic surface coating materials have not been known which have both weatherability and moisture resistance at a high level.
EVA has been used because of 1) long shelf life, 2) relatively good weatherability, 3) adhesiveness to various substrates, 4) ease of cross-linking, and 5) low cost; however, EVA is easily flammable, as are many other transparent organic resins. The flammability is undesirable for application in dense residential areas not only in the case of solar cell modules installed by integration with roofing materials but also in the case of solar cell arrays installed on a frame. Making solar cell modules flame resistant or retardant is predicted to be very important in the manufacture of solar cell modules prevalent in future housing.
Covering the modules with glass would be most suitable for overcoming these problems; thus, sealing solar cells with glass has been extensively tried. However, coating with glass has such problems as lack of flexability, impact resistance, weight reduction, and cost, In addition, even with glass coating, solar cell elements might be exposed on their back side to flame when the backside is not made of heat resistant materials; therefore it cannot be made of combustible material.
In view of these situations, fluororubber may be a candidate for use as a high performance filler for solar cells.
In a report, in 1979, of the Jet Propulsion Laboratory, US Department of Energy, entitled "Investigation of Test Methods, Materials, Properties and Processes for Solar Cell Encapsulation.", use of a fluororubber (Trade name: Viton, manufactured by DuPont) is disclosed for use as a sealing material of solar cell modules. However, with Viton, it is reported in the cited report that yellowing and peeling, due to poor adhesion to the surface material occur in the weatherability test using a Sunshine Weather-O-Meter. Thus, even now, commercial application of fluororubber to solar cell sealing is not employed.
In the case of Viton, effective cross-linking is made with a polyamine or polyol; it is known to add various metal oxides and salts, as acid receptors, which neutralize acidic substances generated in the cross-linking process of the fluororubber, which cross-linking is a type of polyol or polyamine cross-linking. For example, lead oxide or calcium hydroxide is added to cross-link; however, known acid receptors generally including polyol and polyamine are not transparent, thus, this type of cross-linked fluororubber is not suitable for solar cells. That is, the fluororubber that is to be cross-linked with polyamine or polyol is optically opaque when employed as the filler of solar cell modules; thereby, the conversion efficiency of the solar cells is decreased. Even if the acid receptor is transparent, reactions caused by removal of HF occur and the resultant fluororubber becomes brown. In addition, since the cross-linked site is bromine, the weatherability is poor; this is considered to be the reason for yellowing in the accelerated test by the Sunshine Weather-O-Meter mentioned above.
On the other hand, when the above-mentioned fluororubber is used without cross-linking, an optically transparent filler is obtained; however, temperature elevation due to incident light causes a problem. That is, the temperature of solar cell modules located on the roof or provided in a hot area is known to become 80 degrees or higher and the filler undergoes creep when used in such conditions for a long period. In other words, the fluororubber mentioned above moves away from the photovoltaic elements because it is not cross-linked and finally, the covering material may peel off.
Therefore, an object of the present invention is to overcome these problems and provide a solar cell module and production process thereof having a surface covering material, more specifically, a filler resin filling the space between the protective film on the light receiving surface and the semiconductor elements, which covering material is excellent in weatherability and heat resistance, has excellent adhesion to the surface film, limits deterioration of long term performance of the photovoltaic elements by minimizing moisture permeation, has enough rubber elasticity with a sufficient thickness to protect solar cell elements, and is fire retardant or incombustible.